The present invention relates voltage reference circuits, and more particularly to a method and circuit for providing temperature nonlinearity compensation and trimming in voltage reference circuits.
Many integrated circuits require a stable reference voltage for operation. Improved stability in voltage references is being demanded for use in data acquisition systems, voltage regulators, measurement devices, analog-to-digital converters and digital-to-analog converters, to name a few. Voltage references being utilized can include Zener-based references such as buried-Zener references, or bandgap references, which can operate with a lower supply voltage, dissipate less power and provide longer-term stability than that of buried-Zener references.
Ideally, a voltage reference should provide a constant voltage regardless of the circuit temperature or its loading conditions. Buried-Zener references are not available on the most of the modem processes, so bandgap references are most often used when more temperature stability is required. However, any voltage reference has a certain amount of temperature dependence, i.e., the output changes nonlinearly with temperature.
The cause of such nonlinearity is mainly due to non-ideal characteristics of all the reference components. For band gap references the large portion of the nonlinearity is defined by the bow-like non-linearity of the base-emitter voltage (Vbe) of a bipolar transistor with respect to temperature. Generally, any of the nonlinearity characteristics of voltage references can be approximated by a Taylor Row expression, such as Y=a2T2+a3T3+ . . . anTn, where accuracy of the approximation improves with the increasing number of terms.
Many circuits have attempted to implement logarithmic correction of the bandgap reference output voltage, commonly referred to as xe2x80x9ccurvature correctionxe2x80x9d in such circuits. One more well-known and long-used circuit is the xe2x80x9cBrokaw Cellxe2x80x9d as is illustrated in FIG. 1. However, the Brokaw curvature correction technique is only a second-order (T2) correction, whereas real bandgap circuits have a significant amount of higher-order curvature.
Over the last ten years or more, circuits have been developed in an attempt to provide second and third-order approximation of the bandgap curvature. Such circuits have used, for example, temperature-dependent resistors in a Brokaw cell or other similarly modified structures. While such circuits use nonlinear temperature dependence of current or resistance to control the input of the voltage reference, such circuits have become significantly more complicated and costly in order to generate higher order correction terms, and have generally relied on process matching and component stability for permanence on the approximated curves. Since real voltage references deviate significantly from the theoretic Tln(T) characteristic of an ideal bandgap reference, the matching and component selection process can be expensive. Moreover, packaging processes cause additional shifts in both the output voltage and temperature drift, while trimming of the voltage reference circuits after packaging has not yet been implemented.
In accordance with various aspects of the present invention, a method and circuit for temperature nonlinearity compensation and trimming of a voltage reference are configured to provide for two-point independent trimming of each of the curvature coefficients within the Taylor approximation curve.
In accordance with an exemplary embodiment of the present invention, a voltage reference circuit is configured with a translinear circuit having an input current source. The voltage reference circuit comprises a voltage reference having a control input terminal for receiving an output signal control signal and an output terminal for providing an output reference signal. The translinear circuit comprises a translinear unit having a plurality of output currents corresponding to the curvature coefficients of the Taylor row approximation curve, with the output currents coupled to the control input terminal of the voltage reference. The input current source is configured with the translinear unit to be trimmable to a zero value at a nominal temperature.
During trimming of the voltage reference, at a first nominal temperature, the input current source is trimmed to a zero value, and each of the curvature terms of the Taylor approximation will be equal to zero value at the first temperature. At a second temperature the plurality of output currents of the translinear circuit can be measured to enable independent trimming of each of the curvature coefficients such that the output currents of the translinear circuit are made substantially equal to predetermined values. Thus, each of the coefficients of the Taylor approximation curve are independently trimmed to pass through zero at the first temperature and through the predetermined values at the second temperature without regard from one voltage reference circuit to another. As a result, the approximation curve as a whole does not change from one circuit to another as long as the translinear circuit has a stable input/output function. Further, independently trimming for at least two points for each curvature coefficient make the Taylor approximation curve repeatable.